Divisibility
Rules: No, it's not Cheating!
By Lisa Williams
November 10, 2000
These rules are useful
for kids from the time they start division, on. They keep me from using a
calculator and most of the time, I can even work it in my head faster than
the calulator because of these rules. I use them most when reducing awkward
fractions.
To Divide by ... 
Cheat Codes

2 
A number is divisible by 2 if it ends in 0 or an even number.
Example:
40 divided by 2 = 20
3974 divided by 2 = 1987
.

3 
A number is divisible
by 3 if the sum of the digits is divisible by 3.
Example: 321
add 3+2+1= 6 so 321 divided by 3 = 107
2460 add 2+4+6+0= 12 so 2460 divided by 3 = 820



4 
A number is divisible
by 4 if the last 2 digits are both zeros or if they are divisible by 4.
Example:
300 divided by 4 = 75
3548 divided by 4 = 887



5 
A number is divisible
by 5 if it ends in 0 or 5.
Example:
25 divided by 5 = 5
400 divided by 5 = 80



6 
A number is divisible
by 6 if it is even and divisble by 3.
Example: 246
is even and 2+4+6 = 12 so 246 divided by 6 = 41



9 
A number is divisible
by 9 if the sum of the digits is divisible by 9.
Example: 81
add 8 + 1= 9 so 81 divided by 9 = 9
972 add 9+7+2 = 18 so 972 divided by 9 = 18



10 
A number is divisible
by 10 if it ends in 0.
Example: 400
divided by 10 = 40

